The present invention relates generally to the field of photoresists. In particular, the present invention relates to the field of chemically amplified positive-acting photoresists.
Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist-coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable agents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For positive-acting photoresists, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble. In general, photoresist compositions include at least a resin binder component and a photoactive agent.
More recently, chemically-amplified type resists have been increasingly employed, particularly for formation of sub-micron images and other high performance applications. Such photoresists may be negative-acting or positive-acting and generally include many crosslinking events (in the case of a negative-acting resist) or deprotection reactions (in the case of a positive-acting resist) per unit of photogenerated acid. In the case of positive chemically-amplified resists, certain cationic photoinitiators have been used to induce cleavage of certain “blocking” groups pendant from a photoresist binder, or cleavage of certain groups comprising a photoresist binder backbone. See, for example, U.S. Pat. Nos. 5,075,199; 4,968,581; 4,810,613; and 4,491,628 and Canadian Patent Application No. CA 2,001,384. Upon cleavage of the blocking group through exposure of a coating layer of such a resist, a polar functional group is formed, e.g. carboxyl or imide, which results in different solubility characteristics in exposed and unexposed areas of the resist coating layer. See also R. D. Allen et al. Proceedings of SPIE, 2724:334-343 (1996); and P. Trefonas et al. Proceedings of the 11th International Conference on Photopolymers (Soc. of Plastics Engineers), pp 44-58 (Oct. 6, 1997).
While currently available photoresists are suitable for many applications, current resists can also exhibit significant shortcomings, particularly in high performance applications such as formation of highly resolved sub-half micron and sub-quarter micron features.
Consequently, interest has increased in photoresists that can be photoimaged with short wavelength radiation, including exposure radiation of about 250 nm or less, or even about 200 nm or less, and particularly for sub 160 nm wavelengths, particularly 157 nm. Use of such short exposure wavelengths can enable formation of smaller features. Accordingly, a photoresist that yields well-resolved images upon sub 160 nm exposure could enable formation of extremely small (e.g. sub-quarter micron) features that respond to constant industry demands for smaller dimension circuit patterns, e.g. to provide greater circuit density and enhanced device performance.
However, many current photoresists are generally designed for imaging at relatively higher wavelengths, such as I-line (365 nm) and G-line (436 nm) exposures, and are generally unsuitable for imaging at short wavelengths. In particular, prior resists exhibit poor resolution (if any image at all can be developed) upon exposure to these shorter wavelengths. Among other things, current photoresists can be highly opaque to extremely short exposure wavelengths, such as 248 nm and 193 nm, thereby resulting in poorly resolved images.
Typical photoresists suitable for imaging at 248 nm or 193 nm use deprotection chemistry based on derivatives of carboxylic acids or phenolic moieties. Such deprotection chemistry is exemplified by the following reaction schemes: Typical deprotection chemistry, such as the above described phenolic and carboxylic acid-based deprotection schemes, are less suitable for use at wavelengths below about 160 nm because the chemical moieties involved in these schemes absorb radiation strongly in that region of the electromagnetic spectrum.
Alternatively, Feiring et al. in PCT Patent Application Nos. WO 00/17712 and WO 00/67072 disclose a variety of polymer binders for use in photoresists for imaging at 157 nm wavelengths. These and other polymer binders having greater transparency in short wavelength exposure can contain one or more electronegative groups on a monomer unit to reduce the total absorbance of the polymer. Of particular interest among these electronegative groups are fluorinated alcohol substituents as disclosed by Przybilla et al. in German Patent No. DE 42 07 261 and German Patent Application No. DE 42 07 263 because they are sufficiently acidic to allow dissolution in aqueous base developers and reduce the absorbance of the polymer.
Fluorinated alcohols can also be protected by acid cleavable groups. For example, Przybilla et al. in DE 42 07 261 disclose that fluorinated alcohol moieties can be protected by acid cleavable groups such as tri(C1-C4)alkyl methyl, di(C1-C4)alkyl-(C6-C10)aryl-methyl, benzyl, tri(C1-C20)alkyl silanyl, (C1-C20)alkoxycarbonyl, tetrahydropyranyl and tetrahydrofuranyl. In addition, Feiring et al. teach the use of the 3-cyclohexenyl ether and, more generally, alpha-alkoxyalkyl ether moieties as protecting groups for fluorinated alcohols.
For commercially viable photoresists, the choice of a protecting group depends on the following criteria:    1. The chemistry used to substitute fluorinated alcohols with protecting groups should be accomplished with sufficient ease to provide a commercially viable process for manufacturing resist raw materials.    2. The acid catalyzed deprotection reaction should be sufficiently facile to be accomplished near or below the glass transition temperature of the resist but not so facile as to undergo significant deprotection during exposure unless the products of deprotection are nonvolatile. Deprotection that is overly facile and leads to volatile products can, over many exposures, contaminate the objective exposure lens surface.    3. The deprotection reaction should occur in a manner that results in minimal shrinkage of the resist film to avoid mechanical strain that limits resolution and may be associated with the collapse of resist patterns during development.    4. The deprotection group should exhibit minimal absorbance at the exposing wavelength.    5. Byproducts from the deprotection reaction should not interfere with subsequent development of the resist image. In addition, after development and subsequently, the resist deprotection byproducts should leave no residues on the device substrate that could cause defects.
Known protecting groups for fluorinated alcohols do not meet the criteria set forth above. For example, tri(C1-C4)alkyl methyl, di(C1-C4)alkyl-(C6-C10)aryl-methyl and benzyl ethers of fluorinated alcohols do not undergo acid catalyzed deprotection with sufficient reactivity. Further, the di(C1-C4)alkyl-(C6-C10)aryl-methyl and benzyl moieties exhibit high absorbance below 160 nm. Also, tri(C1-C20)alkyl silanyl moieties and silicon containing protecting groups generally can cause post develop or post-etch residues which, in turn, can produce harmful defects on the semiconductor substrate. Additionally, (C1-C20)alkoxycarbonyl protecting groups, such as the tert-butoxycarbonyl group, can generate at least two moles of volatile byproducts per mole of deprotection events. This can lead to resist shrinkage and the volatile alkene byproduct can foul the exposure lens, particularly over a plurality of exposures. Tetrahydropyranyl and tetrahydrofuranyl and methoxymethyl ethers of fluorinated alcohols as well as vinyl ether adducts in general may undergo deprotection during exposure and create volatile byproducts that can foul the exposure lens. In addition, byproducts left in the exposed regions of the film can undergo cationic polymerization. At sufficient concentrations, the resulting polymer can interfere with development. Deprotection chemistry employing 3-cyclohexenyl ether is not sufficiently facile to be accomplished near or below the glass transition temperature of the resist, particularly when using polymers disclosed by Feiring et al.
Additionally, it is desirable to have carboxylic acid or ester functionality in photoresist polymer binders. Such functionality is convenient for functionalization of the polymer and aids in dissolution or removability of the polymer in aqueous developers and strippers. Typically, in polymer binders containing carboxylic acid or ester functionality, including conventional polymer binders for use at sub 160 nm wavelengths, such carboxylic acid or ester functionality is not present on monomer units further containing an electronegative substituent. Thus, typical monomer units containing carboxylic functionality include (meth)acrylic acid, alkyl (meth)acrylates, cyclic olefin monomers substituted with one or more carboxylic acids or esters, and the like.
U.S. Pat. No. 6,057,083 (Taylor et al.) discloses photoresist compositions particularly suitable for imaging at sub 200 nm wavelengths, such as 193 nm. The disclosed photoresist compositions include a polymer binder having an acid labile leaving group selected from optionally substituted fenchyl; optionally substituted phenyl; optionally substituted 3,2,0 bridged system; optionally substituted bridged heteroalicyclic group; and optionally substituted cycloalkyl group having 3 or 4 ring carbon atoms. Such leaving groups are bonded to the polymer binder through a carboxylate ester functionality. The use of such leaving groups on polymer binders suitable for imaging at sub 160 nm wavelengths is not disclosed. Also not disclosed in this patent is the bonding of such leaving groups to a carboxylate ester functionality including one or more electronegative substituents.
U.S. Pat. No. 6,136,501 (Trefonas et al.) discloses photoresist compositions particularly suitable for imaging at short wavelengths, such as 248 and 193 nm. The disclosed photoresist compositions include a polymer binder having a pendant ester group containing a leaving group bonded to the ester carbonyl group the, where the leaving group is an optionally substituted noncyclic alkyl moiety having 6 or more carbon atoms, with at least 2 carbon atoms selected from secondary, tertiary and quaternary carbon atoms, and wherein a quaternary carbon atom is directly bonded to the ester carbonyl group. The use of such leaving groups on polymer binders suitable for imaging at sub 160 nm wavelengths is not disclosed. Also not disclosed in this patent is the bonding of such leaving groups to a carboxylate ester functionality including one or more electronegative substituents.
It is thus desirable to have photoresist compositions that can be imaged at short wavelengths and contain resin binders that undergo facile deprotection without producing volatile byproducts, byproducts that interfere with development or byproducts that produce residues that lead to defects on the device substrate.